Automatic Repeat reQuest (ARQ) denotes a family of protocols used to provide robustness to a communication link, such as a telecommunication link. One example ARQ scheme is denoted Hybrid Automatic Repeat reQuest (Hybrid ARQ or HARQ), and is a technique used, for example, in the UMTS (Universal Mobile Telecommunications System) standard for the HSPA (High Speed Packet Access) protocols, and is being proposed for use in the UMTS LTE (UMTS Long Term Evolution) standard that is under development within the 3GPP (3rd Generation Partnership Project) framework.
Generally in ARQ schemes, the receiver indicates to the sender if a packet has been received in error and, if so, the sender automatically re-transmits the packet. This process may be iterated until the packet is correctly received or until a maximum number of re-transmissions have been carried out.
In a HARQ scheme, the receiver combines the information in the erroneously received packet with the information in the re-transmitted packet and uses the combined information in the demodulation and decoding. This principle of operation is denoted soft combining and enhances the reception performance. The re-transmitted packet in a HARQ scheme may either comprise the same information as it did in the previous transmission, or it may comprise alternative information compared to the previous transmission.
When the re-transmitted packet comprises the same information as it did in the previous transmission, the scheme is denoted chase combining. In chase combining, the combination of information from the erroneously received packet and from the re-transmitted packet may be carried out using either the received symbols before demodulation or the demodulated bits before decoding.
When the re-transmitted packet comprises alternative information compared to the previous transmission, the scheme is denoted incremental redundancy. The packet to be re-transmitted may, for example, be exposed to a different puncturing pattern than in the previous transmission of the packet. This may result in that the re-transmitted packet comprises additional redundancy bits compared to the previous transmission. Alternatively, it may result in that some bits of the previous transmission have been excluded and additional bits have been comprised in the re-transmitted packet. In this way, new redundancy may be added to the information in the received packet for each re-transmission.
An incremental redundancy scheme may typically be based on a low-rate channel code, where only a limited number of the encoded bits are included in the first transmission (i.e., a large number of the encoded bits are punctured). This effectively results in that the first transmission is carried out using a high-rate channel code. If this first transmission of the packet results in an erroneous reception and the packet is to be re-transmitted in response to a re-transmission request, a different puncturing pattern may be used to puncture the packet before re-transmission (i.e., the encoded bits sent in the re-transmission differs from the ones sent in the previous transmission). Thus, successive combining of the information from the transmission and re-transmissions of a packet results in effective channel codes of gradually lower rates.
Hence, in the case of incremental redundancy, each re-transmission is, in general, different from the previous transmission. This leads to that the combination of information from the erroneously received packet and from the re-transmitted packet may not be carried out using the received symbols before demodulation, in contrast to the case of chase combining. Instead, it may be advisable to demodulate each received transmission or re-transmission of a packet separately and buffer the demodulated bits in the receiver. The soft combining may then be carried out implicitly as part of the decoding process. The decoding process may still use soft information to decode the demodulated bits.
In a communication system, transmission resources such as channelization codes in UMTS and physical resource blocks in UMTS LTE may be assigned to a user or a process by a scheduler. A user equipment (UE) may thus be assigned one or several transmission resources. Each base station (or possibly each base station controller) of a communication system commonly comprises a scheduler. Alternatively, each base station may comprise a plurality of schedulers.
Transmission resources are orthogonal to each other in most communication systems. In UMTS/HSPA, for example, the channelization codes are made up by Hadamard-Walsh codes, which are known to be mutually orthogonal. In the uplink of UMTS/HSPA, however, the codes are not orthogonal. Thus, the more codes are used in uplink transmission, the more interference is experienced.
In UMTS LTE, the physical resource blocks (PRB) are confined within a restricted time-frequency interval. Thus, each PRB is orthogonal in time and frequency to every other PRB used in a particular geographical area (cell).
There exist various strategies applicable to ARQ schemes for controlling transmissions of a packet and for determining when a packet in need for re-transmission should be scheduled. For synchronous traffic, which is characterized by a constant offering of data, a scheduler could reserve a large number of transmission resources for packet transmissions in advance. For example, a Voice over Internet Protocol (VoIP) packet may arrive every 20 ms on the average. Hence, it may be reasonable to schedule a transmission of a VoIP-packet for an ongoing VoIP-process in each time interval of 20 ms in advance. This strategy is also known as persistent scheduling or persistent reservation technique, and reduces the control overhead signaling on the link.
However, if a packet needs to be re-transmitted or not, and how many times it needs to be re-transmitted, depends on the error probability of the communication link (that is, the properties of the propagation path). This is a statistical process and may not be predictable. Hence, it is not possible to predict the need for re-transmissions in advance. In persistent scheduling, the re-transmission opportunities are also reserved in advance. Under most (except the worst) channel conditions a large number of these scheduled re-transmission opportunities are not used and the corresponding transmission resources remain empty, which may severely reduce the capacity of the communication link.
An alternative technique for controlling the assignment of re-transmission opportunities is denoted dynamic scheduling. In dynamic scheduling, the re-transmissions are handled by the scheduler in connection to each request for re-transmission. This technique is not as detrimental to the capacity of the communication link as the persistent scheduling technique, but instead it increases the control overhead and the latency. For example, for each re-transmission request in an uplink situation, a scheduling assignment signal may need to be supplied to the scheduler and a scheduling grant message that includes a resource allocation may need to be transmitted to the sender.
It is clear from the above that it is not obvious how to control the use of transmission resources for ARQ re-transmissions efficiently. Thus, there is a need for methods and devices for controlling transmission resources for ARQ processes to optimize the link capacity while introducing moderate or no overhead control signaling and latency.
In U.S. Pat. No. 7,181,666 B2 a method to reduce transmission latency is provided, in which a group of users in a system that employs a re-transmission mechanism such as Automatic Repeat reQuest (ARQ) is divided into multiple sub-groups of users. Transmission intervals are altered among the multiple sub-groups of users.
In US 2004/0013102 A1 a wireless communication system is disclosed, which includes a shared time division multiplexed (TDM) data channel. The TDM data channel can be used for automatic re-transmissions.
Hou C. et al., “Sharing of ARQ slots in Gilbert-Elliot channels”, IEEE Transactions on Communications, vol. 52, no. 12, December 2004, pp. 2070-2072, treat the problem of m transmission slots sharing a pool of n automatic repeat request slots.